(a) Field of the Invention
This invention relates to a new and improved process for the preparation of phosphoric esters and thiophosphoric esters by reaction of a hydroxyl-containing organic compound with phosphorus oxychloride or phosphorus thiochloride.
(b) Description of the Prior Art
Phosphoric esters and thiophosphoric esters which are reaction products of alcohols or phenols with phosphorus oxychloride or phosphorus thiochloride have long been produced for use as agricultural chemicals, resin additives (flameretardant plasticizers), and the like or in the manufacture thereof. For example, tricresyl phosphate (hereinafter referred to as TCP) which is a reaction product of cresol with phosphorus oxychloride was initially developed as a plasticizer for cellulose nitrate and subsequently came into wide use as a plasticizer for polyvinyl chloride. Thus, a variety of processes for the preparation thereof have been proposed in the past.
The preparation of TCP is discussed from the viewpoint of reaction kinetics, for example, in "Kagaku-to-Kogyo," Vol. 4, No. 12, pp. 7-12. It is described therein that the processes for the preparation of TCP which have heretofore been reported in the literature can be classified into four categories:
(a) non-catalytic method, (b) basic catalyst method, (c) acid catalyst method, and (d) metallic salt method.
(a) In the non-catalytic method, the esterification reaction of phosphorus oxychloride is retarded as more chlorine atoms of the phosphorus oxychloride are replaced by phenyl radicals. Moreover, the frequency factor is also reduced so that the rate of formation of substituted products is decreased. Accordingly, high temperatures and long times are required for the completion of the reaction.
(b) The basic catalyst method generally involves the use of tertiary amines. In this case, the rate of formation of mono- and diesters is enhanced. However, such basic catalysts are not effective in promoting the formation of triesters.
(c) The acid catalyst method is based on the use of anhydrous aluminum chloride, iron chloride, magnesium chloride, and the like. For example, when aluminum chloride is used, the rate of formation of di- and triesters is not decreased and, therefore, the reaction can be completed at a relatively low temperature in a short period of time. However, the use of aluminum chloride necessarily induces a Friedel-Crafts reaction which gives rise to undesired compounds having one or more phenyl radicals attached directly to the phosphorus atom.
(d) The metallic salt method is the most typical one that is used in the field of organic synthesis. According to this method, the reaction can be completed at a low temperature in a short period of time, and the product can be obtained in quantitative yield. However, it is described in the literature that this method is not suitable for industrial purposes, in view of its economic disadvantages such as the necessity of converting cresol into a metallic salt.